This invention relates to a process for preparing a thermally stable flame retardant product which is predominant in the bisimide, ##STR1##
N,N'-bis(tetrabromphthalimide) predominant products have been found to be useful flame retardants in polyesters, e.g. polybutylene terephthalate, and other resin formulations. While various processes have been described for the preparation of such bisimide predominant products, almost all produce a product which contains significant amounts of volatile impurities, which impurities contribute to the product's lack of good thermal stability. See, for example, the process of Bonnet et al. U.S. Pat. No. 4,894,187 wherein a N,N'-bis-(tetrabromophthalimide) product is produced which contains up to 80% by weight of a substantially equimolar amount of the impurities, tetrabromophthalic anhydride and N-aminotetrabromophthalimide. The tetrabromophthalic anhydride impurity is especially troublesome as it volatilizes at polymer processing temperatures.